The presence of corrosive products, contaminants and/or ferromagnetic particles, or non-ferrous metallic particles in a lubricating oil are threatening symptoms from the system in which the oil is used because of damage that has, or will occur to the system if deteriorating components are not replaced or repaired, or the oil is not promptly changed.
Many methods and devices have been developed to detect the contamination or breakdown, of oil. One such device, shown in U.S. Pat. No. 4,646,070 issued to Yasuhara, discloses a device for detecting deterioration in lubricating oil which comprises a pair of capacitor electrodes coated with insulating material and positioned in the lubricating oil. The device uses the oil as a dielectric between the sensors to develop a periodic voltage signal across the sensor capacitor, thus determining changes in the dielectric constant indicative of deterioration of the oil. A major weakness of this device and other similar devices is that they do not indicate the type or magnitude of deterioration (e.g., contamination or wear) in the system.
U.S. Pat. No. 5,262,732 issued to A. D. Dickert et al teaches a method and apparatus for simultaneously testing for, and identifying corrosive products, contamination, and ferromagnetic particles as well as non-ferrous wear particles in lubricating oil. Since the apparatus detects the type of products present in oil, a user is able to make a more knowledgeable determination of the conditions causing the deterioration of the oil. The Dickert et al instrument uses a permanent magnet in combination with an electromagnet to attract, orient, and hold ferrous particles against a capacitative surface. Because the Dickert et al instrument does not consume the test sample during the test sequence, multiple tests of the same oil sample may be conducted.
Use of this type of apparatus as might be applied to on-board, continual, real-time analysis of vehicle engine oil, particularly in heavy-duty diesel engines, is considered by the inventors of the present invention to be especially useful in light of the following additional background.
As is known, prolonged operation of a machine, such as an engine, with degraded lubricant can significantly decrease the longevity of the machine. Typically, engine manufacturers recommend engine oil change intervals in terms of either a predetermined calendar period, engine running hours, or distance travelled by a vehicle powered by the engine. These service intervals are usually based upon historical experience gleaned from similar applications. Laboratory oil analysis helps to improve oil drain recommendations, but is costly and the results are often received too late to allow effective corrective action. Thus, the practice of changing oil based on a predetermined recommended schedule may result in replacing oil which has not yet detrimentally degraded. Conversely, in some situations, this practice results in operating the engine with severely degraded lubricating oil. The practice is still often followed, however, due at least in part to the difficulty in establishing criteria for accurately evaluating the quality or condition of the oil. Although satisfactory for some applications, a predetermined oil change schedule is not acceptable for many vehicles because of the wide variations in operating conditions. For example, a predetermined schedule can not anticipate sporadic harsh operating conditions, such as excessive engine idling periods in cold ambient conditions using high sulfur fuel.
Further, extreme operating temperatures, load conditions, contaminants and mechanical failures, alone or in combination, accelerate oil degradation. Such degradation is often manifested as a change in oil color, a change in viscosity, and/or an increase in the presence of soot agglomeration, and water and wear particles, to name a few. Oil quality analysis is discussed in greater detail in Development of an Automatic Engine Oil-Change Indicator System by Shirley E. Swartz and Donaly J. Smolenski, published by the Society of Automotive Engineers, 1987, incorporated herein by reference. Swartz and Smolenski suggest an empirical mathematical model approach based upon current measured engine operating parameters such as engine hours, ambient temperatures, engine oil temperature, and the like, rather than detecting the condition of the oil directly.
A fresh, petroleum-based lubricating oil is primarily composed of hydrocarbon molecules with no net electrical charge which are weakly polar or have a non-polar charge distribution. Fresh mineral oils can be characterized as having a very high electrical resistance and a relatively low dielectric constant (permittivity). These electrical properties change as the oil degrades and becomes contaminated. Specifically, increases in insoluble content, the presence of moisture and acids, or the presence of conductive metallic and non-metallic debris may increase the dielectric constant of an oil, reduce its resistance, or both.
A combined measure of permittivity and resistivity can be made by measuring the AC impedance or effective capacitance (rate of charge divided by the applied potential) across two plates separated by a quantity of oil. An approximate model for the system is an ideal capacitor influenced primarily by permittivity, with a parallel resistance influenced primarily by ionic conduction. Charge mobility not involving conductive particles in a dielectric fluid involves mechanical motion of charged or dipole particles within the fluid. Therefore, system impedance is related to the parameters which describe the hydrodynamics of particles moving in a fluid. These parameters include the oil viscosity, the applied electric and magnetic forces, particle size, and particle shape.